This invention relates to a multi-stage semiconductor memory comprising a plurality of semiconductor memory cells, each loaded with a datum of two or more bits, and more particularly to a method of reading such data from the memory cells and to a multi-stage semiconductor memory device of the type described.
In order to give a large memory capacity to a nonvolatile semiconductor memory device (herein referred to as an ROM), known is a scheme of storing a datum of a plurality of bits in a selected memory cell of the semiconductor memory cells. Such a multi-stage ROM is disclosed, for example, in Japanese Patent Prepublication (A) No. 81,024 of 1978. According to this patent prepublication, ions are selectively injected by using four different masks into a semiconductor substrate to different impurity concentrations in order to enable each semiconductor memory cell to store information of sixteen different kinds, namely, four-bit information. This provides a four-stage ROM with a quarter of semiconductor memory cells of a usual ROM of a common memory capacity.
It is, however, necessary on reading the data not only to sense on or off of the selected memory cell by supply of a read voltage (for example, 5 V) to a selected word line and to a selected digit line of the multi-stage ROM but also to sense a selected threshold level of a plurality of predetermined threshold levels that is given to the selected memory cell in correspondence to a selected one of the different kinds of information. Two typical manners are known for sensing the selected threshold level. A first manner of the two manners is revealed in Japanese Patent Prepublication (A) No. 47,188 of 1993. A second manner is similar partly to the method according to this invention.
In the manner which will later be described in greater detail, the first manner uses successive drive of the selected word line with different read voltages to sense the selected digit line to decide at which of the different read voltages the selected threshold level is exceeded. When the different kinds are four in number, the different read voltages are three in number and may be equal to 2.0 V, 3.5 V, and 5.0 V. In the four-stage ROM, a sensing unit merely senses the selected digit line as regards on or off of the selected memory cell at each of up to three read voltages and is therefore relatively simple in structure. The sensing unit must, however, sense the on or off of the selected memory cell as much as three times in an extreme case. As a consequence, a read voltage supply unit of such a multi-stage ROM must have a complicated structure. Moreover, each word line has a large load capacitance and requires an appreciable length of time before driven stably to one of the different read voltages. As a result, it takes an objectionably long time to read each datum.
According to the second manner, the selected word line is driven to a predetermined read voltage of 3.0 V for example. The sensing unit senses the selected digit line as regards a cell current which flows in the multi-stage ROM through the selected memory cell to which the selected threshold level is given in correspondence to a selected one of the different kinds of information. On reading a datum stored in the selected memory cell, its cell current is compared with a plurality of predetermined constant currents. In order to definitely decide the datum, a greater cell current must be used, such as about twice as strong as a smaller cell current. When the different kinds of information are four in number, the cell current must have three different values of a ratio of 1:4:16. Use of simple circuitry is sufficient for the read voltage supply unit. The cell currents, however, have current values strongly dependent on the selected threshold level. Particularly when an operating voltage is low on putting the multi-stage ROM in operation, the predetermined threshold levels have only a small difference between two consecutive ones of the threshold levels. This makes it unavoidable to use a small voltage difference between two adjacent ones of the constant currents. As a result, the sensing unit undesiredly renders it impossible to read the data.